Uninterruptible power supply systems are generally known. For example, an uninterruptible power supply system is applied to an equipment connected to a commercial alternating current power source. Then, when electric power supply from the commercial alternating current power source to the equipment becomes down by power failure, the uninterruptible power supply system continues the electric power supply to the equipment for a fixed time.
Some uninterruptible power supply systems employs fuel cells. For example, Japanese Patent Application Laid-Open Publication No. 2004-229416 describes a direct current power supply system having a configuration that a rectifier, a storage battery, and a fuel cell device are connected between a commercial power source and a load. In this direct current power supply system, at the time of power failure of the commercial power source, the direct current power of the storage battery is supplied to the load. A control part provided in the fuel cell device detects the output voltage of the storage battery. If the output voltage of the storage battery goes to or below a reference value, the control part starts the fuel cell. In general, the storage battery has a characteristic that the output voltage decreases with decreasing remaining battery charge. The uninterruptible power supply system of the conventional art utilizes this characteristic of the storage battery so that, if the output voltage of the storage battery goes to or below a reference value, the remaining battery charge of the storage battery is recognized as having decreased so that the fuel cell is started.